Single aperture laser radars and related systems often comprise fast optical scanners and chirp modulators. These fast optical scanners in conjunction with optical signal path length differences introduce, due to Doppler shift effects, instantaneous frequency deviations between a transmitted frequency and a received transmitter backscatter frequency. This frequency deviation causes a significant problem when it is desired to filter out, or reject, the backscattered frequency component. An inability to effectively reject the backscattered frequency component may result in, for example, severe range masking effects in a continuously transmitting and receiving laser radar system.
It has been known to employ a complex-vector modulator (CVM) type filter to reduce the backscattered frequency component, such as one disclosed in copending application Ser. No. 06/807,391 (now U.S. Pat. No. 4,731,587 issued Mar. 15, 1988). While providing beneficial filtering results in many applications, in some applications the CVM filter may not be adequate.
For example, the bandwidth of the CVM filter may be inadequate to reject rapidly time-varying frequency shifts induced by fast optical scanning devices in the signal path. Also, the maximum dynamic range of the CVM filter is typically limited to 30-40 dB due to "feedthrough" limitations. Furthermore, the CVM filter is essentially an active, as opposed to a passive, filter and comprises dual feedback paths, making the filter sensitive to level variations in a reference Transmitter Signal (TSIG) input. This TSIG signal is typically a product of the laser local oscillator (LO) and the transmitter laser and is, therefore, essentially a replica of the transmitter signal heterodyned to the first Intermediate Frequency (IF) of the system. Also, the frequency bandwidth rejection characteristics of the CVM filter are not readily changed, nor may it be practical to provide for a frequency bandwidth rejection characteristic that rejects multiple frequency bandwidths.
It is therefore one object of the invention to provide a chirped backscatter filter which has a large bandwidth for rejecting rapidly time-varying frequency shifts caused by optical scanning devices in the signal path.
It is another object of the invention to provide a chirped backscatter filter which has a dynamic range in excess of 30-40 dB.
It is another object of the invention to provide a chirped backscatter filter which has passive, as opposed to active, characteristics thereby rendering the filter less sensitive to level variations in the TSIG input signal.
It is a still further object of the invention to provide a chirped backscatter filter which has easily varied filter rejection bandwidth characteristics.
It is one still further object of the invention to provide a chirped backscatter filter which has, if desired, filter rejection bandwidth characteristics for rejecting a plurality of different frequency bandwidths.